26 HUMAN PHYSIOLOGY. 



vals of digestion, the cells lining the gland are large, clear and highly 

 refractive, and contain a large quantity of mucigen. After secretion has 

 taken place, the cells exhibit a marked change. The mucigen cells have 

 disappeared, and in their place are cells which are small, dark and com- 

 posed of protoplasm. It would appear that the cells, during rest, elaborate 

 the mucigen which is discharged into the tubules during secretory activity, 

 to become part of the secretion. 



Saliva is an opalescent, slightly viscid, alkaline fluid, having a specific 



gravity of 1.005. Microscopical examination reveals the presence of 



salivary corpuscles and epithelial cells. Chemically it is composed ol 



\/ water, proteid matter, a ferment (ptyalin) and inorganic salts. The amount 



secreted in 24 hours is about 2.% Ibs. Its function is twofold : 



1. Physical. Softens and moistens the food, glues it together, and 

 facilitates swallowing. 



2. Chemical. Converts starch into grape sugar. This action is due 

 to the presence of the organic ferment, ptyalin. The change consists in 

 the assumption of a molecule of water. 



Starch. Water. Grape Sugar. 



C 6 H 10 5 + H a O = C 6 H la 6 . 

 NERVOUS CIRCLE OF INSALIVATION. 



AFFERENT OR EXCITOR NERVES. EFFERENT OR MOTOR NERVES. 



1. Lingual branch of 5th pair. I. Auriculo-temporal branch of 5th 



2. Glosso-pharyngeal. pair, for parotid gland. 



2. Chorda tympani, for sub- maxil- 

 lary and sub-lingual glands. 



The centres regulating the secretion are two, viz. : The medulla oblon- 

 gata and the sub-maxillary ganglion of the sympathetic ; the latter acting 

 antagonistically to the former. Impressions excited by the food in the 

 mouth reach the medulla oblongata through the afferent nerves ; motor im- 

 pulses are there generated which pass outward through the efferent nerves. 



Stimulation of the auriculo-temporal branch increases the flow of saliva 

 from the parotid gland ; division arrests it. 



Stimulation of the chorda tympani is followed by a dilation of the blood 

 vessels of the sub-maxillary gland, increased flow of blood (thus acting as 

 a vaso-dilator nerve) and an abundant discharge of a thin saliva; division 

 of the nerve arrests the secretion. 



Stimulation of the cervical sympathetic is followed by a contraction of 

 the blood vessels, diminishing the flow of blood (thus acting as a vaso-con- 

 strictor nerve) and a diminution of the secretion, which now becomes thick 



